Method for photochemically enhancing resolution in photolithography processes

ABSTRACT

In the fabrication of integrated circuits, a polymethyl methacrylate film containing a selected dye and exhibiting a strong dependence on light intensity is photobleached to provide an optical mask to pattern an underlying photoresist layer. While the film is photobleached, the underlying photoresist layer is made to be substantially unaffected by the photobleaching process. When the optical mask is realized, it is used to mask the light-sensitive photoresist layer when the photoresist layer is exposed to light. However, the photobleached layer, which is also sensitive to light, is now in turn made to be substantially unaffected by the exposure process. In this manner, the integrity of the optical mask resolution is maintained at its optimum, and densely integrated circuits can be processed and fabricated.

BACKGROUND OF THE INVENTION

In recent times, fabrication of integrated circuits has resulted inextremely dense circuits For example, such densely integrated circuitshave feature sizes on the order of 1 micron This density is a directresult of achieving high resolution masks through projectionphotolithography during the fatrication process.

In prior art projection photolithography, an image, that is, apreselected pattern, is projected onto a layer of photoresist, and theexposed portions of the photoresist layer absorbs the energy of theprojected image. The photoresist layer then is chemically developed andprocessed to form a mask of the projection image, and the mask is usedin subsequent etching processes. If the image projected containsextremely small, sharply defined features, the mask produced alsodefines small features, and the integrated circuits fabricated from themask can be made correspondingly dense. Unfortunately, two phenomena inthe early prior art act to limit the smallness of features and thesharpness of the projection image: standing waves and nonuniformtopology of the photoresist layer. The disadvantages of these phenomenahave been discussed in detail by Tai et al. in "Submicron opticallithography using an inorganic resist/polymer bilevel scheme," J. Vac.Sci. Technol., 17(5), Sept./Oct. 1980, pp. 1169-1177. Thesedisadvantages are now summarized.

In the early prior art photolithography photoresist layer, light wavesare reflected at the surfaces of the layer to interfere with incidentwaves. The interference between incident and reflected waves causesstanding waves within the layer, and the nodes of the standing wavesbecome sites of minimal energy absorption to cause differing lightexposure or dose within the layer. As a result, the mask pattern createdfrom the photoresist layer does not sharply correspond to the projectionimage. The resolution of the image in the mask consequently becomeslimited in part by the extent of such standing waves.

In forming a layer of photoresist, the profile, or topography, of thelayer is generally not uniform. In other words, the layer normally has,microscopically speaking, undulations of valleys and peaks to result ina layer of varying thickness. Because of the varying thickness, theabsorption of energy when an image is projected will also not beuniform, and the image developed in the photolithography mask willcorrespondingly be not uniformly sharp. The resolution of the maskconsequently also becomes limited by the extent of nonuniformity of thephotoresist layer topography.

To counter these limiting effects of the early prior artphotolithography process, the method represented by FIG. 1 and disclosedby Barlett et al. in "A Two Layer Photoresist Process in a ProductionEnvironment," Proceedings of SPIE, Vol. 394, 1983, pp. 49-56, is used.In this prior art process, double thickness layers 12 are formed byapplying a dye-photoresist layer 22 to an integrated circuit substrate10 and then superposing a photoresist layer 33 on it. Thedye-photoresist layer 22 typically contains a mixture of a dye to absorbreflection waves and in that way eliminate or minimize the standing waveeffect. Furthermore, a double thickness layer 12 tends to distributemore uniformly over the integrated circuit substrate 10; this in turnreduces the non-uniform topology effect. In this double thickness layerprocess, the photoresist layer 33 is exposed and developed to provide arelatively planar mask for patterning the underlying dye-photoresistlayer 22. The close proximity of the mask to the dye-photoresist layerin combination with the reflection absorbing dye allows creating a maskof enhanced resolution from the dye-photoresist layer 22 for use insubsequent etching. This process, however, has an inherent disadvantage:control of line width is very difficult because of the high sensitivityof the process to any variations in image exposure dose and photoresistdevelopment time and temperature. Consequently, the actual dimensionsrealized in the process vary considerably from the intended projectiondimensions to result in integrated circuits having much less resolutionthan the projection circuits.

West et al. in "Contrast Enhancement--A Route to Submicron OpticalLithography," Proceedings of SPIE, Vol. 394, 1983, pp. 33-38, discloseanother refinement in the prior art to overcome the disadvantages inearly prior art photolithography for improving the resolution in thephotolithographic process. West et al. teach that a photobleachablelayer is formed over a photoresist layer. An image is then bleached intothe photobleachable layer by irradiating the photobleachable layer witha light of high intensity. As the photobleachable layer is bleached andbecomes progressively a transparent optical mask, the irradiating lightcontinues through the transparent optical mask and simultaneouslyexposes the photoresist layer. In other words, this method creates anoptical mask capable of high resolution to be used for exposing aphotoresist sublayer. But this prior art scheme requires that the lightsource remains sufficiently intense in one area to both photobleach thephotobleachable layer into an optical mask and expose the photoresistlayer with sufficient light to allow the photoresist layer to bedeveloped later but not to further degrade the lightsensitive opticalmask. Herein lies its disadvantage: this prior art process, though animprovement over the early prior art, presents a conflicting requirementof irradiating an optical mask and illuminating a photoresist mask witha common light source. This process, then, requires a careful control oflight dose in the process so that only a dose sufficient to expose aphotoresist layer throught an optical mask is used; otherwise, theoptical mask is further degraded and resolution in the integratedcircuit fabrication process is lost.

SUMMARY OF THE INVENTION

In the preferred embodiment of the invention, the disadvantage of theprior art of requiring a careful control of the common light source toobtain an optical mask and expose an underlying photoresist layer tolight is eliminated. Accordingly, a photobleachable layer is formulatedso that when it is irradiated and photobleached to realize the opticalmask, the underlying photoresist is essentially unaffected by theirradiation. Likewise, when the photoresist layer is exposed to lightusing the optical mask obtained, the optical mask in turn is essentiallyunaffected by the process. In this manner, the optical mask obtainedfrom the photobleachable layer is not degraded by the processing of thephotoresist layer.

Specifically, a layer of photoresist is applied to an integratedsubstrate, for example, an integrated circuit wafer. Next, a layercontaining a dye exhibiting a highly nonlinear photobleaching responseto incident fluence intensity, for example, a mixture of polymethylmethacrylate (PMMA) and acridine, is applied over the photoresist layer.Because of the high concentration of dye that may be required, the dyeis dispersed in the polymer layer. Alternatively, it can be covalentlybound to the polymer for forming a relatively uniform mixture. Apreselected pattern is then projected onto the layer of nonlinearphotobleachable dye, a PMMA/acridine layer in the illustratedembodiment, with a light source having a uniform intensity profile, forexample, an excimer laser-illuminated (ELI) projection camera. Becauseof the dye compound used, a pulse of irradiation of proper intensity isapplied such that the whole pulse is absorbed in the photobleachingprocess; essentially none of the light intensity is allowed to fall andaffect the photoresist sublayer. The photobleachable layer is convertedinto an optical mask, and it is used to mask the photoresist sublayer. Adose of low intensity light of sufficient duration is next used toexpose the photoresist layer. Because it is low intensity and because ofthe judicious choice of a photobleachable dye, the optical mask alreadyformed remains unaffected while the photoresist layer is being exposed.The net result is a substantially undegraded optical mask of highresolution.

Again judiciously choosing the proper dye, dyes of anthracenederivatives in this instance, a photobleachable layer is irradiatedwithout essentially affecting a photoresist sublayer by irradiating itin the proper environment, an environment of oxygen in this instance.Then, when the photoresist sublayer is to be exposed to light after thephotobleachable layer has been converted to an optical mask, theenvironment is changed, for example, to a non-oxygen environment, andthe photobleached layer becomes impervious to the light passing throughit to irradiate the photoresist sublayer. Again, a substantiallyundegraded optical mask is preserved to achieve high resolutionconfigurations of integrated circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the double layer process in photolithography in the priorart.

FIG. 2 shows a nonlinear photobleaching layer of PMMA and acridine and aphotoresist layer in accordance with an illustrated embodiment of theinvention.

FIG. 3 shows the photobleaching step using laser irradiation.

FIG. 4 shows the resultant photobleached layer ready for use as anoptical mask for a photoresist sublayer.

FIG. 5 shows an illustrated embodiment of the invention for obtainingthe transmission data of Table I.

FIG. 6 shows the trend of transmission of the illustratedphotobleachable layer with incident fluence.

DETAILED DESCRIPTION OF THE INVENTION

To appreciate the underlying technology in the novel method disclosed, adescription of the photobleaching of thin polymer films by excimer laseris first described. Then a description of the application of suchphotobleaching to photolithography to enhance resolution in accordancewith the invention follows.

In the illustrated embodiment of the invention of FIG. 5, imageresolution enhancement in photolithography results from nonlinearresponses of strongly intensity-dependent photochemistry in thin polymerfilms using 249-nm (KrF) laser irradiation at high irradiances. A laserbeam 55 from an excimer laser 52 is directed through a metal aperture 54and is focussed by a lens 56. A second aperture 57, which can be a mask,is centered and placed in the diverging region of the beam 55. Thissecond aperture 57 is used to define the incident fluence forphotobleaching a sample 14 containing a thin polymer film 40 which has ahighly nonlinear photochemical response to light. The sample 14, a0.1-mm-thick fused silica disc 11 coated with the film 40, is placedimmediately behind this second aperture 57. The aperture 57 and sample14 can be moved with respect to the lens 56 to vary the incidentfluence. At the highest fluence used, the increase in beam diameterbetween the aperture 57 and the film 40 is less than 10%. Photodiodesignals (not shown) for both incident and transmitted energies arerecorded on a storage oscilloscope. The absolute values of the fluencesare measured and the intensity of the photobleaching pulse is calculatedto provide the data in Table I.

                  TABLE I                                                         ______________________________________                                        I (MW/sq cm)    No. of Shots                                                                             % T                                                ______________________________________                                        4.2             2          71                                                 4.0             2          64                                                 3.4             3          59                                                 3.3             3          19                                                 3.2             4          8.9                                                3.0             7          19                                                 2.7             17         5.3                                                2.6             22         0.7                                                2.4             49         0.2                                                2.3             172        0.8                                                2.2             900        0.3                                                2.1             1400       0.3                                                2.0             1500       0.3                                                ______________________________________                                         Number of pulses to obtain measurable transmission for various incident       intensities (averaged), and % T of the final pulse. For the 2.3-2.0 MW/sq     cm data, the pulses were delivered at 10 Hz; others were spaced by less       than 5 sec. For the 4.2-2.7 MW/sq cm data, 2% was the minimum detectable      level; after that it was 0.1%.                                           

The sample 14 consists of a layer 40 of approximately 20 wt. % acridinein polymethyl methacrylate (PMMA) spun to a nominal thickness of 1micron. FIG. 6 shows the transmission T of low-intensity pulses, thatis, pulses less than 2 MW/sq cm, versus the fluence of a single priorbleaching pulse, which is totally absorbed for all intensities. That thebleaching is intensity-dependent and not fluence-dependent isdemonstrated by the fact that T does not change due to the lowerintensity pulses for many hundreds or thousands of pulses; at 2 MW/sq cmit takes about 2200 pulses (at 10 Hz) to get 1% T, and twice that numberto get 2%.

Table I shows the dependence of T on integrated dose for variousintensities. The shot-to-shot variations in the laser output, which mayamount to 15%, make a delivery of a series of pulses of identicalintensity impossible. Thus the intensity given is an average, and thestrong dependence of bleaching on intensity in the transition region,i.e., the region between about 3 and 4 MW/sq cm, limits the utility ofsuch an average. In particular, the intensity of the first pulse for the3.4 and 3.3 MW/sq cm data is lower than the average. This fact accountsfor the need of two bleaching pulses rather than one as is expected fromFIG. 6. Nevertheless, the trend is clear and indicates the remarkablesharpness of the onset of bleaching 60.

Because of the sharp onset of photobleaching in the nonlinearintensity-dependent film 40, the film 40 can be used to create a mask 42for patterning a photoresist layer 20 in an integrated circuitprocessing sequence as illustrated in FIGS. 2-4 and yet essentially notaffecting the photoresist layer 20 during the process. By photobleachinga projection image 50 onto the film 40 with a high intensity pulse or aseries of high intensity pulses calculated to be wholly absorbed by thedye in the photobleaching process, a layer 42 having a highly nonlineartransmission profile corresponding to the image 50 is createdessentially without affecting a photoresist sublayer 20 adjacent to it.Light 7 of a lower intensity, an intensity sufficiently low to haveessentially no effect on the photobleached film 42, is then passedthrough the film 42 to the photoresist sublayer 20 in sufficient dose toalter the exposed areas of the sublayer 20. The sublayer 20 is nextdeveloped and processed, and the remaining regular integrated circuitetching and processing steps are then carried out. As an example, if aprojection image has an overall contrast Imax/Imin of 1.5, and anintensity ratio near the line edge I(0)/I(-0.0125) of 1.08, whereI(-0.0125) refers to the intensity 0.0125 micron inside the nominallymasked region, and I(0) refers to the line edge, then an I(0)=3.2 MW/sqcm will give a low intensity T(0)=5%, approximately. And ifI-(0.0125)=2.96 MW/sq cm, T will be less than 0.01%. Thus, the film 42,remaining unaltered with the irradiation of the photoresist sublayer 20,is essentially a perfect threshold detector for an image 50 which isvery close to the diffraction limit of resolution. In the integratedcircuit processing steps, the pattern transfer into the planarizingresist 20 using the photobleached thin film 42 as a mask can occureither in the same alignment step or in a subsequent blanket exposure toavoid any loss in throughput.

An alternate embodiment of the invention alters the photobleachingenvironment to prevent any degradation or effect of an already formedoptical mask when an underlying photoresist layer is irradiated. Toaccomplish this, an anthracene derivative dye is dispersed in a polymerlayer to form a photobleachable layer. This photobleachable layer, whichhas been formed over an underlying layer of photoresist material, isconverted into a high resolution optical mask. An imaging exposure ofthe photobleachable layer is made by irradiating the layer with apreselected pattern with light in an oxygen environment. Absorption oflight in the presence of oxygen results in transformation of the dye toa non-absorbing form in the photobleachable layer. Since the dye in theillustrated embodiment is an anthracene derivative (to whichketocoumarins may or may not be added to sensitize the reaction of thedye), the reaction in the photobleaching process is a self-sensitizedphoto-oxidation. The excited anthracene, which is in a singletelectronic state, transfers part of its excitation energy to oxygen andconverts to a triplet state to produce a singlet oxygen molecule. Thissinglet oxygen molecule is highly reactive toward the ground-stateanthracene; it causes the photobleaching. And with the addition ofketocoumarins, which have a near unity quantum yield of triplets, anoverall photo-oxidation yield of nearly unity is achieved. Because ofthis dependence of the dye on oxidation in the photobleaching process,altering the irradiation process of the underlying photoresist layer toa non-oxygen environment causes the photobleached layer to becomeimpervious to the irradiation dose of the photoresist layer. Theintegrity of the optical mask remains unchanged, and high resolutionintegrated circuits are achieved.

I claim:
 1. A method of processing integrated ciruits comprising the steps of:depositing a first layer of photoresistive material on a substrate; depositing a second layer containing a mixture of a polymer and a dye upon said first layer, said dye having a nonlinear photobleaching response to the intensity of an incident fluence; irradiating said second layer with a preselected pattern to convert said second layer to an optical mask of said preselected pattern without substantially effecting said first layer; and illuminating said second layer to transfer said preselected pattern to said first layer without substantially further effecting said second layer.
 2. The method as in claim 1, wherein said dye is acridine.
 3. The method as in claim 2, wherein said polymer is polymethyl methacrylate (PMMA).
 4. The method as in claim 3, wherein said step of irradiating is irradiating with an excimer laser. 